1. Field of the Invention
The invention generally relates to variable gain amplifiers and applications of the same. In an embodiment, the variable gain amplifier is used in a set-top control box for the delivery of cable television service to a customer.
2. Related Art
Variable gain amplifiers are known in the art. What is needed is a more linear, less costly approach to providing variable gain.
The present invention is a variable gain amplifier having a plurality of differential pair amplifiers that are sequentially switched on and off to provide variable amplification. A similar plurality of logic decoder circuits compares an automatic gain control voltage and generates logic signals to switch certain amplifiers on and off, depending on the amplitude of the automatic gain control voltage. The invention covers the embodiment wherein ten differential pair amplifiers amplify an unattenuated cable TV signal when full amplification is desired. As less amplification is desired, the first amplifiers are switched out and others are switched in, where the newer amplifiers have attenuated inputs. The switching continues until only ten amplifiers remain on. At this point, if less amplification is desired, amplifiers are switched out starting with the least attenuated amplifier, but no replacement amplifiers are switched in. There will always be a minimum number of amplifiers in the circuit. In one embodiment, the minimum number is three.